#include<stdio.h>

int main(void) {
	int N;
	long long int a[51];

	scanf("%d", &N);
	
	a[1] = 1;
	a[2] = 2;
	for (int i = 3; i <= N; i++) {
		a[i] = a[i - 1] + a[i - 2];
	}

	printf("%lld\n", a[N]);

}